Process optimization for high fracture toughness of maraging steel rings formed by mandrel forging

Maraging steel rings are generally processed through ring rolling as the technique provides near-net shape seamless rings with uniform properties and minimum material wastage. A technology development programme to establish mandrel forging as an alternate route for making rings of 3.0 m class was initiated, but serious limitations in processing heavy-duty rings were uniformity in microstructure, mechanical properties and above all reliability posed technical challenges. 18 Ni 1800 grade maraging steel rings of 3.0 m class having H/D≈0.11H/D≈0.11, d/D≈0.95d/D≈0.95 and S/D≈0.025S/D≈0.025 (HH: height, dd: inner diameter, DD: outer diameter, SS: thickness) have been made through mandrel forging using a 6000 tons hydraulic press.An appropriate forging sequence was evolved to impart maximum working in the range of 800–1000 °C, and further punching operation was modified such that material waste in form of slug could be reduced. This resulted in the material consumption parameter ‘QQ’ (ratio of weight of forged ring to proof machined ring) in the range of 2.5 to 3.0, which was much less than the requirement adopted by industries based on the empirical relation evolved in the past for decades, especially for thin walled rings.Control over thermo-mechanical processing resulted in the breaking of embrittling particles like TiC/Ti(CN) formed during melting and ensured their uniform distribution throughout the matrix. A suitable heat treatment schedule was adopted to get uniform recrystallised grains that resulted in high fracture toughness.The paper presents the details of the above illustrated approach for the successful development of mandrel forging technique for making large size thin walled maraging steel rings. This study emphasizes the need of modification in the conventional empirical relation for mandrel forging.